Here's another article on kayaking Llyn Brianne, this one courtesy of the Daily Mail.
This one interviews Shaun Baker, one of the first people to ever kayak down it. He says:
I can't believe how stupid I was and I would urge anyone else not to go near it. If you don't hit the bottom dead straight and fly over the wall, you can either break your back on impact, break your neck on the steel sill or just drown. The boat just went ballistic. I spun 360 degrees clinging on to my paddles for dear life. I must have worn three inches off the blades and if I'd lost them, I'd have worn my fingers down to the first joint just trying to steer. If you don't steer and you hit that sill, you'll just shear your head off. I managed to get it right just as I hit the bottom.
Suddenly, I was being forced under and thrown around like a pair of socks in a washing machine. I must have been stuck under for about 40 seconds before I surfaced on the edge. The whole episode was just very scary. I just look back on it and think: "I shouldn't have got out of that."
The article also has a short interview with Gary Connery who bicycled down it.